Being skinny on top with extra fat around the bottom can give natural protection against heart disease and diabetes, according to a study which looked at 700 middle-aged twins.
But women who are "apples" - those who store fat around their middles - are at greater risk from these diseases and need to be more careful.
"Storing fat around your bottom is better than around your stomach," Dr Tim Spector, director of the Twin Research Unit told BBC1's Watchdog HealthCheck. "We believe this is because the fat around your bottom is slower in the way it is turned over and therefore less likely to appear in the blood stream."
The study, which is published in the Journal of Clinical Endocrinology and Metabolism, also found that 60 per cent of body fat is determined by your genes and no amount of dieting can change this.
Doctors measured the amount and position of the twins' body fat and found identical twins always shared the same shape, suggesting the tendency is almost entirely genetic.
The finding explains why some women find it hard to lose weight by dieting and why others discover it is almost impossible to shift areas of fat from their body, no matter how hard they exercise.
"Diet is unlikely to affect where the fat goes for most people," said Dr Spector. "This is rather pre-ordained and that's why if you look at your mother or other older relatives you can see what your body shape is likely to be."
At the third international conference on eating disorders in London, doctors warned that women with eating disorders were seven times more likely to fracture their bones because of increased susceptibility to the brittle bone disease osteoporosis.
Moreover, with girls falling victim to dieting and anorexia at increasingly earlier ages, children as young as ten are at risk of stunted growth and irreversible loss of bone density.
Dr Bryan Lask, consultant psychiatrist at Great Ormond Street Hospital, which runs the only NHS specialist unit for children with eating disorders in the country, warned that healthy eating messages had become misinterpreted so that children were terrified of getting fat.
Thirteen years ago the hospital used to get two referrals a year for children with eating disorders. Now it gets four per week.Reuse content